Implement split view browsing controls

[Andresshamis]

Meridian Explorer automation, posting through @Andresshamis.

Priority: P2
Type: feature / accessibility / testing / architecture
Ready status: ready

Problem statement

Meridian has split-view data models and persistence-boundary handling, but users cannot create, view, use, or close a split browsing layout. The command-bar action enum still includes splitActiveTab, yet the action is intentionally unavailable and the browser content area renders only one active WKWebView at a time.

Why it matters for Meridian

Split view browsing is part of Meridian's product scope and an important sidebar-first workflow: users should be able to compare pages without leaving the current space. Leaving only dormant split-view models creates misleading architecture coverage without an actual browser capability.

Evidence

• Sources/MeridianCore/Models/SplitViewLayout.swift defines split-view layout metadata, and BrowserTab.splitViewID plus BrowserSessionSnapshot.splitViews persist the shape.
• Sources/MeridianCore/Services/SessionPersistenceBoundary.swift filters invalid/private split views, so persistence already accounts for split metadata.
• Sources/MeridianCore/Stores/BrowserStore.swift exposes splitViews but has no public API to create, select, update, or close a split layout; performBrowserAction(.splitActiveTab) returns false and the user message is Split view is not available yet.
• Sources/MeridianCore/Views/BrowserWindowView.swift owns a single WebViewState, and Sources/MeridianCore/Views/Browser/BrowserContentView.swift renders a single active WebViewHost or StartPageView; splitViews is not used in the view layer.
• Sources/MeridianCore/Services/CommandRouter.swift defines .splitActiveTab, but allActionSuggestions does not include a split-view suggestion and availability filtering hard-disables it.
• Docs/FeatureChecklist.md lists split-view models as implemented and Split view UI as not yet implemented.
• Issue Wire command bar browser actions #26 and merged PR Wire command bar browser actions #28 intentionally deferred split-view action activation; searches found no open dedicated split-view issue beyond that closed command-bar slice.

Affected files/components

• Sources/MeridianCore/Models/SplitViewLayout.swift
• Sources/MeridianCore/Models/BrowserTab.swift
• Sources/MeridianCore/Stores/BrowserStore.swift
• Sources/MeridianCore/Views/BrowserWindowView.swift
• Sources/MeridianCore/Views/Browser/BrowserContentView.swift
• Sources/MeridianCore/WebKit/WebViewState.swift
• Sources/MeridianCore/WebKit/WebViewHost.swift
• Sources/MeridianCore/Services/CommandRouter.swift
• Sources/MeridianCore/Models/CommandBarResult.swift
• Tests/MeridianBrowserTests/BrowserStoreTests.swift
• Tests/MeridianBrowserTests/CommandRouterTests.swift
• Tests/MeridianBrowserTests/SessionPersistenceBoundaryTests.swift
• Docs/FeatureChecklist.md
• Docs/TestPlan.md

Expected behavior

A user can split the active tab into a two-pane browsing layout within the current space, interact with either pane, keep each pane's WebKit navigation state separate, and return tabs to normal single-pane browsing without losing tab metadata or violating profile/private browsing boundaries.

Proposed direction

Implement a narrow first split-view slice for two horizontal panes. Add store-level APIs to create a split from the selected tab, add or select the companion tab, assign both tabs to a SplitViewLayout, select the active pane/tab, and close/unsplit safely. Update the content view to render two WebViewHost panes, each with its own WebViewState, while keeping WebKit integration behind the existing boundary. Wire a visible control and the command-bar splitActiveTab action only when an active tab can be split.

Acceptance criteria

• A selected regular/pinned/favorite tab can enter a two-pane split layout with a companion tab in the same space and profile.
• Split creation updates splitViews, both tabs' splitViewID, selected tab/pane state, and session persistence through the existing persistence boundary.
• The browser content area renders both panes side by side with separate WebViewState instances and no cross-pane back/forward/reload leakage.
• Users can focus/select either pane and run back, forward, reload/stop, close tab, and command-bar actions against the focused pane.
• Users can close or unsplit a split layout; remaining tabs keep valid section/folder membership and no stale splitViewID remains.
• Private/ephemeral profile split metadata remains excluded from durable persistence through SessionPersistenceBoundary.
• Split commands/suggestions are unavailable when there is no active tab or when the active tab cannot be split, with only generic non-sensitive messaging.
• Docs/checklist distinguish implemented first-slice split view behavior from future enhancements such as drag-resizing, vertical orientation, and more than two panes.

Dependencies/blockers

• Not blocked by issue Add Xcode app project and UI test host #4 for store, command routing, persistence-boundary, and SwiftUI view unit coverage.
• End-to-end UI/WebKit fixture tests should be added or tracked after issue Add Xcode app project and UI test host #4 / PR Add Xcode app project and UI test host #12 provides a signing-capable UI test host.
• Coordinate with issue Add user controls for essentials and pinned tabs #17 so tab placement controls and split membership do not create duplicate ownership across Essentials, Pinned, regular Tabs, folders, and split panes.

Suggested tests

• BrowserStoreTests for creating a split from the selected tab, assigning a companion tab, preserving same-space/same-profile membership, selecting panes, closing one pane, and unsplitting.
• Persistence tests proving public split layouts round-trip and private/ephemeral split metadata is filtered before disk writes.
• CommandRouterTests for split-view aliases/results and unavailable-action behavior.
• View-level tests or follow-up UI tests proving the focused pane receives command-bar and toolbar actions.
• Regression tests for closing a split tab from Essentials/Pinned/regular/folder contexts without duplicate tab IDs or stale split IDs.